Abstract

Background

Research has reported that pregnant women and mothers become forgetful.
However, in these studies, women are not recruited prior to pregnancy, samples
are not representative and studies are underpowered.

Aims

The current study sought to determine whether pregnancy and motherhood are
associated with brief or long-term cognitive deterioration using a
representative sample and measuring cognition during and before the onset of
pregnancy and motherhood.

Method

Women aged 20–24 years were recruited prospectively and assessed in
1999, 2003 and 2007. Seventy-six women were pregnant at follow-up assessments,
188 became mothers between study waves and 542 remained nulliparous.

Results

No significant differences in cognitive change were found as a function of
pregnancy or motherhood, although late pregnancy was associated with
deterioration on one of four tests of memory and cognition.

Conclusions

The hypothesis that pregnancy and motherhood are associated with persistent
cognitive deterioration was not supported. Previous negative findings may be a
result of biased sampling.

Many pregnancy guidebooks, the popular press and websites counsel pregnant
women on the possibility of short-term memory problems during pregnancy,
describing the condition as ‘baby brain’ or ‘placenta
brain’.1–3
These views are supported by scientific research evidence and systematic
reviews.4–20
For example, a recent systematic review found that pregnant women perform more
poorly than non-pregnant women on memory and other cognitive
tests.4 Similar
deficits are reported in motherhood in
cross-sectional19
and longitudinal
studies13,18
(but see Crawley et
al).17
However, the animal-model literature paints a radically different picture of
the effects of pregnancy and motherhood on cognitive capabilities. Kinsley
et al21
reported better spatial learning and memory during pregnancy in rats, as have
others.22 These
improvements persist. In motherhood, parous rats compared with nulliparous
rats navigated mazes more
efficiently,23
experienced less anxiety and fear, as indexed by levels of stress hormones in
the blood and by behaviour in open
fields.24,25
Although the tests used to measure cognitive functioning in rodents and humans
differ markedly, it is surprising that outcomes of the human and rat research
appear to be at such odds, given that ‘most mammals share similar
maternal behaviours, which are probably controlled by the same brain regions
in both humans and
rats’.25 This
suggested to us that the effect of pregnancy or motherhood on cognitive
abilities may not have been adequately tested. No study of human mothers has
collected or examined data on cognitive performance prior to pregnancy, the
samples recruited to the studies were effectively convenience samples, with
many women recruited as volunteers from prenatal classes and matched to
non-pregnant ‘friends’ and, although studies examined for the
possibility of brief impairments immediately pre- and post-birth, few studies
examined long-term effects, months or years after pregnancy or motherhood. The
current study sought to determine whether pregnancy and motherhood are
associated with cognitive deterioration using a study that measured baseline
cognitive performance before pregnancy and motherhood, and used a large
representative sample. Hypotheses were framed in terms of deficits. To test
our first hypothesis (hypothesis one) that pregnancy leads to impaired
cognitive functioning, we compared women pregnant at wave two with women not
pregnant at or before wave two on change in cognitive function from wave one
to wave two. Analogously, change from wave two to wave three was compared
between those pregnant at wave three and those not pregnant at or before wave
three. To assess short-term effects we examined the effects of length of
pregnancy on cognitive performance as a function of stage of pregnancy
(1–4 months v. 5 months or more). To investigate whether the
birth of a child (i.e. motherhood) impaired cognitive capacity (hypothesis
two), we compared women who became pregnant between waves one and two and who
were, therefore, new mothers at wave two with women not pregnant at or before
wave two. Similarly, we compared women who became pregnant between waves two
and three and who were new mothers at wave three with women not pregnant at or
before wave three. Four areas of cognition were assessed. These were cognitive
speed, working memory, immediate and delayed recall.

Method

Participants

The Personality and Total Health (PATH) Through Life Project is a
prospective longitudinal narrow-age cohort community survey concerned with
health and well-being. Three cohorts were recruited in 1999 (20–24
years, 40–44 years and 60–64
years).26 The
sample for this study consisted of women from the youngest cohort. In this
cohort, 2404 individuals aged 20–24 years were recruited from the
electoral roll with a recruitment rate of 58.6%. Of these, 1241 were women who
completed the baseline measurement in 1999. Subsequently, 1126 and 1058 women
completed the first and second follow-up in 2003 and 2007. This represented a
follow-up rate of 91% and 85% respectively. Of the 115 participants who were
not interviewed beyond the first wave, 83 refused or were unable to be
interviewed for medical reasons, 30 could not be located and 2 died between
the waves. Only women who had completed at least both waves one and two were
considered in the present analyses. Ethics approval was obtained from the
Australian National University’s Human Research Ethics Committee.

Over the 8 years of the study, 76 were pregnant at follow-up (2003 or 2007)
and 188 women became mothers (but were not pregnant at the time of the
interview). Only primigravidae and first-time mothers were studied because
multiple pregnancies and mothering of more than one child would introduce
additional factors. Individuals becoming pregnant or becoming mothers between
1999 and 2003 were considered separately to women becoming pregnant or mothers
between 2003 and 2007. In the analyses the two waves were compared separately
because, over the 8-year period of the study, well-documented changes in IQ,
as demonstrated in standard IQ scales such as Wechsler Adult Intelligence
Scale,27 would be
expected as a result of maturation, practice effects and additional education.
Further, participants who did not become pregnant or progress to motherhood at
any time point may have had characteristics that were different from those not
yet pregnant. The PATH survey has a broad focus and provided no cue that
cognitive performance with respect to either pregnancy or motherhood was a
focus of attention.

Survey procedure

Individuals selected at random from the electoral roll were sent a letter
informing them of the survey and that an interviewer would contact them soon
to see if they wanted to participate. If a person agreed to participate, the
interviewer arranged to meet them at some convenient location, usually the
participant’s home or the Centre for Mental Health Research at the
Australian National University. Most of the interview was self-completed on a
palmtop or laptop computer. However, testing by the interviewer was required
for the physical tests and for some of the cognitive tests.

Cognitive tests

Four domains of cognitive functioning were assessed using tests that are
sensitive to change: cognitive speed, working memory, immediate recall and
delayed recall.

Cognitive speed

Mental speed was measured with the Symbol–Digit Modalities Test
(SDMT), which asks the participant to substitute as many digits for symbols as
possible in 90
s.28

Working memory

Working memory was assessed with the Digits Backwards subtest of the
Wechsler Memory
Scale,29 which
presents participants with series of digits at the rate of one per second and
asks them to repeat the digits backwards.

Immediate and delayed recall

These were assessed with the first trial of the California Verbal Learning
Test,30 which
involves recalling a list of 16 nouns. The interval between immediate and
delayed recall was occupied by a test of grip strength.

Cognitive change scores were calculated for each test by subtracting wave
one scores from wave two scores.

Predictor or control variables

Education

Educational attainment was measured using six questions concerning the full
spectrum of past and current primary (elementary), secondary and tertiary
educational attainment. Responses to these questions were coded into a single
measure corresponding to the number of years of education. For the purposes of
the analyses, education was then categorised into four groups: 0–12
years, 13 years (i.e. high school), 14–15 years and 16 years or
more.

Depression

Depression was measured by the Goldberg Depression
Scale.31 The
Goldberg Depression Scale consists of nine items, which are rated with a ‘
yes/no’ response. Total scale scores are calculated by summing
the number of ‘yes’ responses.

Anxiety

Anxiety was measured by the Goldberg Anxiety
Scale.31 The
Goldberg Anxiety Scale consists of nine items, which are rated with a ‘
yes/no’ response. Total scale scores are calculated by summing
the number of ‘yes’ responses.

Other

Antidepressant medication or anxiolytic medication was assessed by asking
whether participants were ‘taking any depression medication’
(yes/no), or ‘taking anxiety medication’ (yes/no). Participants
were also asked their marital status (married/de facto married, not married),
and whether they had sleeping difficulties, using one item ‘sleeping
poorly’ (yes/no).

Cohorts

We sought to maximise available data at each wave and to use appropriate
groups for comparison. Change in those pregnant at waves two and three were
compared with change in contemporaneously non-pregnant women in separate
contrasts. Seven relevant patterns of pregnancy and motherhood were identified
and subgroups formed on the basis of these
(Fig. 1). Groups one and two
consisted of women who were either pregnant at the first wave of the study or
who had been so beforehand. Because no information was available about the
cognitive status of these women before they became pregnant or mothers these
women could not be included in the study. Groups three to six comprise women
who became pregnant at different stages of the study. Group seven are women
who had never been pregnant up until the last occasion of measurement. The
groups were variously used to test hypotheses as outlined in
Fig. 1.

Group 1: Mothers at entry; group 2: pregnant wave 1; group 3: new mothers
wave 2; group 4: pregnant wave 2; group 5: new mothers wave 3; group 6: new
mothers wave 3; group 7: never pregnant. Additional groups not shown in the
figure: 115 women who dropped out post-wave 1 and 149 women who experienced
multiple pregnancies between waves of data collection.

H1 refers to hypothesis one that pregnancy is associated with
impaired functioning. Two contrasts compared women pregnant at the time of
assessment (P) with those women not (yet) pregnant
(P̄). The first contrast (W2–W1) involved
women pregnant at wave 2, the second (W3–W2), women pregnant at wave 3.
H2 refers to hypothesis two that motherhood leads to impaired
cognitive functioning. Two contrasts compared women who had become mothers
between a previous and the next assessment (M) with women who were not (yet)
mothers (M̄). The first contrast (W2–W1)
involves women who became mothers at wave 2, the second (W3–W2), women
who became mothers at wave 3.

Statistical analyses

One-way analyses of variance (ANOVAs) were used to test baseline
differences between each of the relevant subgroups for each of the cognitive
variables. Mixed model repeated measures
ANOVA32 with group
(as defined in Fig. 1) and wave
as factors were fitted to each variable. Within-participant variation was
modelled using an unstructured covariance matrix. The outcomes of conventional
tests of each main effect and the interaction were not relevant to this study.
Testing each hypothesis involved specifying a two degree of freedom contrast
composed of the comparison appropriate subgroups for wave one to wave two
change and an analogous comparison for wave two to wave three change. Results
from each comparison were also available, enabling exploration of effects
present only for wave two or for wave three. The numbers of participants with
missing data for each comparison are given in the footnote to the
corresponding tables. Sensitivity analyses indicated that, given the size of
relevant subgroups, individual contrasts would have 80% power to detect
between-group differences in the range 0.3 to 0.5 standard deviations.

To examine the effects of pregnancy and recency of motherhood on cognitive
functioning we categorised pregnant women into early and late pregnancy. To
examine the effects of recent transition to motherhood, we divided women into
those with babies under 12 months at the time of testing and those with older
infants. Choice of categorisations was moderated by sample size
considerations. Significance was set at P = 0.05.

Results

One-way analyses of variance (ANOVAs) did not reveal baseline differences
between the relevant sample groups on the tests of cognitive function (Tables
1 and
2). To examine the effects of
potential confounding variables, the relationship of each cognitive outcome
with age, education, depression, anxiety, taking medication for anxiety or
depression, marital status and ‘sleeping poorly’ was investigated
(time-varying for each occasion of measurement). Only education was identified
as a significant covariate for all of the cognitive factors (all
P<0.001). However, further investigations comparing the adjusted
and unadjusted models and estimated means for the outcome variables clearly
demonstrated that the effect of education was negligible. Therefore, the
analyses reported below were not adjusted for education (or any of the other
potential covariates).

Means and standard deviations for new mother and non-mother
(comparison) subgroups (hypothesis two)

The pre-specified contrasts within mixed model repeated measures ANOVAs
found no significant differences in cognitive change from waves one to two
between those who were pregnant and those who were not on tests of cognitive
speed (SDMT) and memory recall (immediate recall, delayed recall)
(Fig. 2 and
Table 3). There was one
significant effect in favour of non-pregnant women on working memory (Digits
Backwards subtest) between waves two and three. Those who became pregnant at
wave three experienced a greater decline in their Digits Backwards score than
those who remained non-pregnant (mean difference in change scores –
0.640, P = 0.037). However, the aggregate effect over the two
waves was not significant and if correction had been made for multiple
testing, the effect would not be significant.

There were no significant differences in cognitive change between waves one
and two for those who had become mothers between waves and those who had not.
This was also the case for cognitive change between waves two and three
(Fig. 3 and
Table 4). The mean cognitive
scores for each sample group across time can be seen in
Fig. 3. Performance improved in
all groups.

Secondary analyses examining stage of pregnancy and time since birth were
also undertaken. Women pregnant at wave two and at wave three were divided
into those only recently pregnant (5 or more months remaining) and those in
later stages (last 4 months of pregnancy). Women in each of these groups were
compared separately with the non-pregnant groups on each measure. The only
effect found was on the SDMT for women in the later stages of pregnancy.
Performance on this test fell from the previous wave by 2.60 points and 2.79
points for women pregnant at waves two and three respectively, whereas
non-pregnant women recorded an improvement (2.23 points at wave two) or
stability (0.13 point drop at wave three). These contrasts were significant
(t687.7 = 2.000, P = 0.046 and
t646.5 = 2.138, P = 0.033). There were no effects
for working memory or immediate or delayed recall. Secondary analysis of the
motherhood data compared recent (infant less than 12 months) with established
motherhood (older). There were no significant differences, and numbers
precluded additional analysis of women immediately post-birth.

Discussion

Findings from the study

In short, no negative cognitive impacts of pregnancy and motherhood were
observed in the full sample in this representative prospective study, except
for the Digits Backwards subtest at wave three. Pregnant women at wave three,
but not wave two, remembered approximately one digit backwards fewer than
prior to pregnancy. However, this isolated finding would not remain
significant under adjustment for multiple testing and was not significant when
both waves were combined. These results provided no support for the
study’s hypotheses with respect to long-term cognitive change as a
function of pregnancy or motherhood. In secondary analyses, we did find that
women in later pregnancy in both waves were poorer on the SDMT but not on any
tests of memory functioning.

Reasons for the discrepancy with extant literature

Although we found an isolated effect of late pregnancy on speed of
cognitive performance in women in late pregnancy, findings from this
prospective study are not consistent with the bulk of the literature, which
reports various forms of cognitive deficit in both pregnancy and motherhood.
One interpretation of the inconsistency is that the findings from previous
studies are biased. This could be because of the recruitment of volunteer
mothers who may differ in significant ways from ‘average’ pregnant
women, being more concerned or anxious about the effect of pregnancy on their
cognitive status or more depressed or sleep deprived. Alternatively, control
volunteers may have differed from the pregnant groups in ways other than their
non-gravid status. They may have a greater investment in cognitive performance
than the pregnant women. Previously reported effects have generally been
modest and could well have resulted from relatively subtle biases to which
samples of convenience and non-randomly assigned groups are at risk.

A second explanation is that more cognitive deficits do exist, but that the
tests used failed to detect them. This is unlikely as the measures employed
were the same or similar to those previously used with reported effects for
working
memory7,8,12
and
speed.14,16
It remains possible that cognitive tests that reflect fluctuations in
attention might reveal more subtle differences. Recently, Rendell and Henry
have argued that pregnant women exhibited increased difficulties in
implementing delayed intentions in daily life, and that these deficits emerged
only outside of laboratory
environments.8
Moreover, tests in the present study, like the majority of previous research,
are not designed to detect whether any potential deficits were because of
motivational or dispositional differences between pregnant and non-pregnant
women. Some commentators have suggested that pregnant women may be less
disposed to undertake cognitive testing, may regard it as less important than
control women, and thus perform more poorly because of poor motivation rather
than diminished
capacity.11 A
limitation of this epidemiological study is that the tests in the current
battery did not assess these specific, narrow abilities.

Limitations of our study

In interpreting the results the time between testing occasions needs to be
considered. For the pregnant women, the pre-pregnancy assessments were up to 4
years before the testing during pregnancy. Women were at various stages of
pregnancy at the time of testing. For these reasons, it is important to
distinguish between short-term and longer-term effects. With the exception of
one effect at wave two, our primary analyses indicated that there were no
longer-term average effects of pregnancy on cognitive performance if all
stages of pregnancy were combined. Our secondary analyses, which separated
groups into those early and late in pregnancy, did find an effect for women
late in pregnancy on a speed task within both waves, but this deficit was not
found on three tests of memory. This finding underscores the importance of
examining effects as a function of stage of pregnancy. Achieving this in
unselected studies will be difficult because of the small numbers of women at
each stage of pregnancy in even the largest studies. Regardless of this, the
specific finding of a speed effect in late pregnancy requires replication.

For tests involving new mothers, the interval between not being a mother
and becoming one was variable. For instance, some women had been mothers for
periods of days whereas others had been mothers for years. There is no clear
indication from the research literature as to when deficits might be likely to
arise or to peak. Some studies have found memory deficits 32 weeks
postpartum.13 Other
studies have reported that the cognitive deficits are no longer apparent after
short intervals of
days.12 We found no
correlation between the age of the child (i.e. period of motherhood) and
cognitive deficits in supplementary analyses.

This study is the largest of its kind. Nevertheless it is limited in its
ability to detect small effects because of the size of the pregnant and new
mother groups. Steps were taken to maximise power and we did detect
significant change in cognition (largely improvements) over the period. We
were able to assess and exclude or control the effects of depression, anxiety,
medication, sleep problems, age and education on the test performance. In the
absence of the possibility of mimicking animal studies and randomly assigning
women to pregnancy or non-pregnancy conditions, the design of the present
trial is as robust as possible in the context of human experimentation.

Finally, one of the weaknesses of our study was our inability to link
cognitive change with biological changes associated with pregnancy. We were
unable to look at the effects of contraceptive use in mothers and non-mothers.
These analyses require targeted rather than epidemiological investigation. Our
study, however, had the advantage of being a large population-based sample
with pre-test measures, features that are not present in studies with
retrospective recruitment.

Implications

We did not find that outcomes for the gravid rat held for the human female.
In particular we did not find significant cognitive improvements in
functioning in pregnant women or in mothers relative to controls as have been
found in earlier animal work. In this context it is important to acknowledge
that human and animal studies do differ substantially, in that domains of
cognition are different (for example water mazes in comparison to verbal
recall), and that improvements in spatial functioning in animals are not
universally found, with some studies finding deficits in rats during the third
trimester.32
Nonetheless, there were no indications of improvement in our study. Moreover,
although cognitive tests will differ for humans and rats, tests are similar in
that they reflect hippocampal functioning (see for example Lye et
al)34 so some
degree of consistency might have been expected. Whether the differences
between humans and animals are a result of differing biological substrates,
the social environment or their interplay remains to be explored. Perhaps,
more importantly from the perspective of mothers to be and those caring for
them, we were also not able to establish substantial or consistent cognitive
deficits. Except in a brief period in later pregnancy, these findings
challenge the common myth that women develop ‘placenta brain’ or ‘
baby brain’. We found no deficits on memory tests in particular.
Since both women and their partners believe that women experience cognitive
deficits in pregnancy (see Christensen et
al),11 women
and their partners need to be encouraged to be less automatic in their
willingness to attribute common memory lapses to the salient causal factor of
a growing or new baby. Obstetricians, general family doctors and midwives may
need to use the findings from this study to promote the view that ‘
placenta brain’ is not inevitable, and that perceptions of
impairment may reflect emotional or other unknown factors. Not so long ago
pregnancy was ‘confinement’ and motherhood meant the end of career
aspirations. Our results challenge the view that mothers are anything other
than the intellectual peers of their contemporaries.

Funding

The PATH Through Life Project is funded by
NHMRC Grant number
418039. H.C. is funded by
NHMRC Fellowship number
525411 and L.L. by NHMRC
PhD Scholarship number 3451048.

Acknowledgments

The authors would like to thank K. J. Anstey, A. F. Jorm, P. J. Butterworth
P. A. Jacomb and K. Maxwell for contributions to the design of the PATH
Through Life Project. Amelia Gulliver assisted in developing the reference
list.